Fuel cells are electric power generation apparatuses with little emission of substances, a high energy efficiency, and a low load on the environments. Therefore, they have again been in the limelight along with the increased concerns for global environmental preservation in recent years. As electric power generation apparatuses for relatively small scale decentralized power generation facilities and transportable bodies such as automobiles and ships, the fuel cells are expected to be future power generation apparatuses. Further, it is expected that the fuel cells are to be disposed in compact type mobile apparatuses such as cellular phones and personal computers in place of the secondary batteries such as nickel-hydrogen batteries and lithium ion batteries.
With respect to polymer electrolyte fuel cells (hereinafter, sometimes referred to also as PEFC), direct type fuel cells in which fuel such as methanol is directly supplied (hereinafter, sometimes referred to also as DFC) are also spotlighted in addition to conventional cells using hydrogen gas as fuel. Although the DFC has a low output as compared with a conventional PEFC, the fuel is a liquid and no reformer is required, so that the energy density is increased and the power generation duration per single charge is long.
A polymer electrolytic fuel cell generally is composed as a cell unit by forming a membrane electrode assembly (MEA) composed of electrodes, an anode and a cathode, which cause reaction of generating electricity, and a polymer electrolytic member to be a proton conductor between the anode and the cathode and sandwiching the MEA with separators. In this case, each electrode is composed of an electrode substrate (also called as a gas diffusion electrode or an electric collector) for promoting gas diffusion and collecting electricity (or supplying electricity) and an electrode catalytic layer where electrochemical reaction is actually carried out. For example, in the case of an anode of PEFC, fuel such as hydrogen gas is reacted in the catalytic layer of the anode to generate proton and electron and the electron is transmitted to the electrode substrate and the proton is transmitted to the polymer electrolytic member. Therefore, the anode is required to have good gas diffusivity, electron conductivity, and proton conductivity. On the other hand, in a cathode, an oxidizing gas such as oxygen and air is reacted with the proton transmitted from the polymer electrolytic member and the electron transmitted from the electrode substrate to produce water in the catalytic layer of the cathode. Therefore, the cathode is required to efficiently discharge produced water in addition to the gas diffusivity, electron conductivity, and proton conductivity.
Particularly, with respect to electrolytic membranes for DFC using organic compounds such as methanol as fuel among solid polymer electrolyte fuel cells, in addition to the properties required for the electrolytic membranes for conventional PEFC using hydrogen as fuel, fuel permeation suppression is also required. The fuel permeation in the electrolytic membrane is called as fuel cross-over (FCO) or chemical short to cause a problem of decrease of fuel cell output and energy capacity.
Further, in a direct fuel cell, properties different from those of conventional PEFC using hydrogen gas as fuel are required. That is, with respect to the direct fuel cell, in the anode, fuel such as an aqueous methanol solution is reacted in the catalytic layer of the anode to generate proton, electron, and carbon dioxide and the electron is transmitted to the electrode substrate, the proton is transmitted to the polymer electrolyte, and the carbon dioxide passes the electrode substrate and is release to the outside of the system. Therefore, in addition to the required characteristics of the anode of a conventional PEFC, permeability of fuel such as an aqueous methanol solution and discharge property of carbon dioxide are also required. Further in the cathode of the direct fuel cell, in addition to the reactions similar to those in a conventional PEFC, reaction of fuel such as methanol passed through the electrolytic membrane and an oxidizing gas such as oxygen or air is caused in the catalytic layer of the cathode to produce water. Therefore, the water to be produced is more than that produced in a conventional PEFC and it is required to further efficiently discharge water.
Conventionally, perfluoro type proton conductive polymer membranes represented by Nafion (trade name, manufactured by Du Pont de Nemours & Co.) have been used as polymer electrolytic membranes. However, these perfluoro type proton conductive polymer membranes have high permeation quantities of fuel such as methanol in the direct type fuel cells and thus there are problems that the cell output and the energy capacity are insufficient. Further, the perfluoro type proton conductive polymers are every expensive in cost due to used of fluorine.
For that, non-fluoro type proton conductive polymer electrolytes are desired in markets and some developments of polymer electrolytic membranes base on non-fluoro type polymers have been tried.
For example, in 1950's, styrene type cation exchange resins have been investigated. However, they have been found insufficient in the strength as membranes in the state for use of general fuel cells and consequently, they have failed to give sufficient cell lives.
Fuel cells using sulfonated aromatic polyether ether ketones as electrolytes have been investigated. For example, it is disclosed (reference to Non-patent Document No. 1) that aromatic polyether ether ketones (hereinafter, sometimes abbreviated as PEEK) hardly soluble in organic solvents are made soluble in organic solvents and easy to form membranes by sulfonation to a far extent. However, these sulfonated PEEK are improved also in hydrophilicity and become water-soluble or deteriorated in the strength at the time of water absorption. The polymer electrolytic fuel cells generally produce water as a byproduct by the reaction of fuel and oxygen and in DFC, water is contained in the fuel in almost all cases and therefore, in the case particularly such sulfonated PEEK become water-soluble, they are not suitable to be used as electrolytes for fuel cells as they are.    [Non-patent Document No. 1]: Polymer, 1987, vol. 28, 1009.
Non-patent Document No. 2 describes sulfonated compounds of PSF (UDELP-1700) and PES which are aromatic polyether sulfones (reference to Non-patent Document No. 2). There is description that the sulfonated PSF completely become water soluble and cannot be evaluated as electrolytes. Meanwhile, although the sulfonated PES do not become water soluble, they have a problem of high water absorption and therefore, cannot be expected to be highly effective to suppress fuel cross-over.    [Non-patent Document No. 2]: Journal of Membrane Science, 83 (1993) 211-220.
Further, Non-patent Document No. 3 describes sulfonated compounds of polyphosphazenes as phosphorus polymer-based polymer proton conductors. However, the sulfonated polyphosphazenes are considerably hydrophilic in the main chains themselves and the water contents are too high to expect them to have a high fuel cross-over suppression effect.    [Non-patent Document No. 3]: Journal of Applied Polymer Science, 71 (1999) 387-399.
Further, a variety of other types of polymer electrolytic membranes produced by introducing anionic groups into non-fluoro aromatic polymers have been proposed (Patent Documents Nos. 1 and 2, Non-patent Document No. 1).    [Patent Document No. 1]: US Patent Application laid-open No. 2002/91225    [Patent Document No. 2]: U.S. Pat. No. 5,403,675    [Non-patent Document No. 4]: Journal of Membrane Science, Vol. 197, 231-242 (2002).
However, these conventional polymer electrolytic membranes become easy to take water in the inside if the introduction amounts of the ionic groups to obtain high conductivity and have a defective point that the cross-over of fuel such as methanol is significant. The polymer electrolytic membranes contain low freezing point water in a large quantity in the membranes and unfreezable water at a low ratio and therefore, it is supposed that fuel such as methanol is easy to pass through the low freezing point water and the fuel cross-over becomes significant.
Patent Document No. 3 discloses polymer electrolytic materials comprising sulfonated polyether type copolymers containing fluorene components. However, the document does not sufficiently description of groups effective for shutting the fuel or of membrane formation methods and according to follow up experiments by inventors, membrane formation is difficult by the method described and no polymer electrolytic membrane is formed.
Further, there are descriptions of polymer electrolytic materials of sulfonated polyether type copolymers containing both fluorene component and phenylene component in Examples 19 and 24 of Patent Document No. 4. However, the fluorene component is introduced only at 20% by mole and the production method and the membrane formation method are different from those of the invention, so that the swelling to the fuel is significant and the cross-over of fuel is considerable and thus the polymer electrolytic materials are not practical for use and the polymer electrolytic materials have a low unfreezable water ratio.    [Patent Document No. 3]: Japanese Patent Application Laid-Open No. 2002-226575    [Patent Document No. 4]: Japanese Patent Application Laid-Open No. 2002-524631.
Further, composite membranes of proton conductive polymers and other polymers are also proposed. For example, composite membranes comprising sulfonated polyphenylene oxide and polyvinylidene fluoride (Patent Document No. 5) have been known. Also, composite membranes comprising sulfonated polystyrene and polyvinylidene fluoride (Patent Document No. 6) have been known. However, the polymer electrolytic membranes described in these documents are membranes of blended polymers of ion conductive polymers and polyvinylidene fluoride and easy to cause significant phase separation structure in μm order owing to bad compatibility of the polymers and thus it has been difficult to satisfy both of high conductivity and fuel cross-over suppression simultaneously. In the polymer electrolytic membranes, low freezing point water and bulk water exist in inter-phases and the ratio of the unfreezable water in the electrolytic membranes is low, so that it is supposed to be difficult to suppress the fuel cross-over.
Further, membranes of composites of proton conductive polymers and copolymers of siloxanes having nitrogen atom-containing groups and metal oxides have been known (Patent Document No. 7). Also, composites of Nafion (trade name, manufactured by Du Pont de Nemours & Co.) and siloxanes have been known (Non-patent Documents Nos. 5 and 6). However, since the membranes described in these documents use Nafion, perfluoro type proton conductive polymers, even if the membranes are composite membranes with other polymers, it is difficult to satisfy both of high proton conductivity and low fuel cross-over simultaneously.
Further, ion exchange materials obtained by polymerizing compositions containing monomers having unsaturated bonds and monomers capable of introducing crosslinking structure after impregnation of porous substrates with the compositions and then sulfonating the produced polymerization products (reference to Patent Document No. 8). However, in the case of using the membranes for direct methanol type fuel cell (hereinafter, also referred to as DMFC), although it takes a long time to carry out sulfonation, the proton conductivity is insufficient and it is difficult to obtain proton conductivity high enough for practical use of the DMFC.    [Patent Document No. 5]: U.S. Pat. No. 6,103,414    [Patent Document No. 6]: Japanese Patent Application Laid-Open No. 2001-504636    [Patent Document No. 7]: Japanese Patent Application Laid-Open No. 2002-110200    [Patent Document No. 8]: Japanese Patent Application Laid-Open No. 2003-12835    [Non-patent Document No. 5]: Polymers, Vol. 43, 2311-2320 (2002)    [Non-patent Document No. 6]: Journal of Material Chemistry, Vol. 12, 834-837 (2002).
In these conventional techniques, there are problems that the electrolytes to be obtained are expensive: that waterproofness (anti-swelling) is insufficient and therefore, the strength is insufficient or fuel cross-over is significant: and that the oxidation resistance and radical resistance are inferior.
The invention aims to provide a polymer electrolytic material excellent in proton conductivity and also excellent in the fuel shutting property and accordingly to provide a polymer electrolytic fuel cell with a high efficiency.